Editing IPCC:AR6/WGI/Chapter-7 (section)

=== 7.5.3 Estimates of ECS Based on Paleoclimate Data ===

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Estimates of ECS based on paleoclimate data are complementary to, and largely independent from, estimates based on process-based studies ( [[#7.5.1|Section 7.5.1]] ) and the instrumental record ( [[#7.5.2|Section 7.5.2]] ). The strengths of using paleoclimate data to estimate ECS include: (i) the estimates are based on observations of a real-world Earth system response to a forcing, in contrast to using estimates from process-based modelling studies or directly from models; (ii) the forcings are often relatively large (similar in magnitude to a CO <sub>2</sub> doubling or more), in contrast to data from the instrumental record; (iii) the forcing often changes relatively slowly so the system is close to equilibrium; as such, all individual feedback parameters, α x , are included, and complications associated with accounting for ocean heat uptake are reduced or eliminated, in contrast to the instrumental record. However, there can be relatively large uncertainties on estimates of both the paleo forcing and paleo global surface temperature response, and care must be taken to account for long-term feedbacks associated with ice sheets ( [[#7.4.2.6|Section 7.4.2.6]] ), which often play an important role in the paleoclimate response to forcing, but which are not included in the definition of ECS. Furthermore, the state-dependence of feedbacks ( [[#7.4.3|Section 7.4.3]] ) means that climate sensitivity during Earth’s past may not be the same as it is today, which should be accounted for when interpreting paleoclimate estimates of ECS.

AR5 stated that data and modelling of the Last Glacial Maximum (LGM; Cross-Chapter Box 2.1) indicated that it was ''very unlikely'' that ECS lay outside the range 1°C–6°C ( [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] ). Furthermore, AR5 reported that climate records of the last 65 million years indicated an ECS 95% confidence interval of 1.1 to 7.0 °C.

Compared with AR5, there are now improved constraints on estimates of ECS from paleoclimate evidence. The strengthened understanding and improved lines of evidence come in part from the use of high-resolution paleoclimate data across multiple glacial–interglacial cycles, taking into account state-dependence ( [[#7.4.3|Section 7.4.3]] ; [[#von%20der%20Heydt--2014|von der Heydt et al., 2014]] ; [[#Köhler--2015|Köhler et al., 2015]] , 2017, 2018; [[#Friedrich--2016|Friedrich et al., 2016]] ; [[#Snyder--2019|Snyder, 2019]] ; [[#Stap--2019|Stap et al., 2019]] ) and better constrained pre-ice-core estimates of atmospheric CO <sub>2</sub> concentrations ( [[#Martínez-Botí--2015|Martínez-Botí et al., 2015]] ; [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#de%20la%20Vega--2020|de la Vega et al., 2020]] ) and surface temperature ( [[#Hollis--2019|Hollis et al., 2019]] ; [[#Inglis--2020|Inglis et al., 2020]] ; [[#McClymont--2020|McClymont et al., 2020]] ).

Overall, the paleoclimate lines of evidence regarding climate sensitivity can be broadly categorized into two types: estimates of radiative forcing and temperature response from paleo proxy measurements, and emergent constraints on paleoclimate model simulations. This section focuses on the first type only; the second type (emergent constraints) are discussed in ( [[#7.5.4|Section 7.5.4]] .

In order to provide estimates of ECS, evidence from the paleoclimate record can be used to estimate forcing (Δ ''F'' ) and global surface temperature response (Δ ''T'' ) in Equation 7.1, Box 7.1, under the assumption that the system is in equilibrium (i.e., Δ ''N'' = 0). However, there are complicating factors when using the paleoclimate record in this way, and these challenges and uncertainties are somewhat specific to the time period being considered.

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==== 7.5.3.1 Estimates of ECS from the Last Glacial Maximum ====

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The LGM (Cross-Chapter Box 2.1) has been used to provide estimates of ECS (see Table 7.11 for estimates since AR5; [[#Sherwood--2020|Sherwood et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ). The major forcings and feedback processes that led to the cold climate at that time (e.g., CO <sub>2</sub> , non-CO <sub>2</sub> greenhouse gases, and ice sheets) are relatively well-known ( [[IPCC:Wg1:Chapter:Chapter-5#5.1|Section 5.1]] ), orbital forcing relative to pre-industrial was negligible, and there are relatively high spatial resolution and well-dated paleoclimate temperature data available for this time period ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] ). Uncertainties in deriving global surface temperature from the LGM proxy data arise partly from uncertainties in the calibration from the paleoclimate data to local annual mean surface temperature, and partly from uncertainties in the conversion of the local temperatures to an annual mean global surface temperature. Overall, the global mean LGM cooling relative to pre-industrial is assessed to be ''very likely'' from 5 to 7 °c ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] ). The LGM climate is often assumed to be in full equilibrium with the forcing, such that Δ ''N'' in Equation 7.1, Box 7.1, is zero. A calculation of sensitivity using solely CO <sub>2</sub> forcing, and assuming that the LGM ice sheets were in equilibrium with that forcing, would give an Earth System Sensitivity (ESS) rather than an ECS (see Box 7.1). In order to calculate an ECS, which is defined here to include all feedback processes except ice sheets, the approach of [[#Rohling--2012|Rohling et al. (2012)]] can be used. This approach introduces an additional forcing term in Equation 7.1, Box 7.1, that quantifies the resulting forcing associated with the ice-sheet feedback (primarily an estimate of the radiative forcing associated with the change in surface albedo). However, differences between studies as to which processes are considered as forcings (for example, some studies also include vegetation and/or aerosols, such as dust, as forcings), means that published estimates are not always directly comparable. Additional uncertainty arises from the magnitude of the ice-sheet forcing itself ( [[#Stap--2019|Stap et al., 2019]] ; [[#Zhu--2021|Zhu and Poulsen, 2021]] ), which is often estimated using ESMs. Furthermore, the ECS at the LGM may differ from that of today due to state-dependence ( [[#7.4.3|Section 7.4.3]] ). Here, only studies that report values of ECS that have accounted for the long-term feedbacks associated with ice sheets, and therefore most closely estimate ECS as defined in this chapter, are assessed here (Table 7.11).

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==== 7.5.3.2 Estimates of ECS from Glacial–Interglacial Cycles ====

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Since AR5, several studies have extended the [[#Rohling--2012|Rohling et al. (2012)]] approach (described above for the LGM) to the glacial–interglacial cycles of the last approximately 1 to 2 million years ( [[#von%20der%20Heydt--2014|von der Heydt et al., 2014]] ; [[#Köhler--2015|Köhler et al., 2015]] , 2017, 2018; [[#Friedrich--2016|Friedrich et al., 2016]] ; [[#Royer--2016|Royer, 2016]] ; [[#Snyder--2019|Snyder, 2019]] ; [[#Stap--2019|Stap et al., 2019]] ; [[#Friedrich--2020|Friedrich and Timmermann, 2020]] ; see Table 7.11). Compared to the LGM, uncertainties in the derived ECS from these periods are in general greater, due to greater uncertainty in global surface temperature (due to fewer individual sites with proxy temperature records), ice-sheet forcing (due to a lack of detailed ice-sheet reconstructions), and CO <sub>2</sub> forcing (for those studies that include the pre-ice-core period, where CO <sub>2</sub> reconstructions are substantially more uncertain). Furthermore, accounting for varying orbital forcing in the traditional global mean forcing and response energy budget framework (Box 7.1) is challenging ( [[#Schmidt--2017b|Schmidt et al., 2017b]] ), due to seasonal and latitudinal components of the forcing that, despite a close-to-zero orbital forcing in the global annual mean, can directly result in responses in annual mean global surface temperature ( [[#Liu--2014|Liu et al., 2014]] ), ice volume ( [[#Abe-Ouchi--2013|Abe-Ouchi et al., 2013]] ), and feedback processes such as those associated with methane ( [[#Singarayer--2011|Singarayer et al., 2011]] ). In addition, for time periods in which the forcing relative to the modern era is small (interglacials), the inferred ECS has relatively large uncertainties because the forcing and temperature response (Δ ''F'' and Δ ''T'' in Equation 7.1, Box 7.1) are both close to zero.

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==== 7.5.3.3 Estimates of ECS from Warm Periods of the Pre-Quaternary ====

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In the pre-Quaternary (prior to about 2.5 million years ago), the forcings and response are generally of the same sign and similar magnitude as future projections of climate change ( [[#Burke--2018|Burke et al., 2018]] ; [[#Tierney--2020a|Tierney et al., 2020a]] ). Similar uncertainties as for the LGM apply, but in this case a major uncertainty relates to the forcing, because prior to the ice-core record there are only indirect estimates of CO <sub>2</sub> concentration. However, advances in pre-ice-core CO <sub>2</sub> reconstruction (e.g., [[#Foster--2016|Foster and Rae, 2016]] ; [[#Super--2018|Super et al., 2018]] ; [[#Witkowski--2018|Witkowski et al., 2018]] ) mean that the estimates of pre-Quaternary CO <sub>2</sub> have less uncertainty than at the time of AR5, and these time periods can now contribute to an assessment of climate sensitivity (Table 7.11). The mid-Pliocene Warm Period (MPWP; Cross-Chapter Box 2.1 and Cross-Chapter Box 2.4) has been targeted for constraints on ECS ( [[#Martínez-Botí--2015|Martínez-Botí et al., 2015]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ), due to the fact that CO <sub>2</sub> concentrations were relatively high at this time (350–425 ppm) and because the MPWP is sufficiently recent that topography and continental configuration are similar to modern-day. As such, a comparison of the MPWP with the pre-industrial climate provides probably the closest natural geological analogue for the modern day that is useful for assessing constraints on ECS, despite the effects of different geographies not being negligible (global surface temperature patterns; ocean circulation). Furthermore, the global surface temperature of the MPWP was such that non-linearities in feedbacks ( [[#7.4.3|Section 7.4.3]] ) were relatively modest. Within the MPWP, the KM5c interglacial has been identified as a particularly useful time period for assessing ECS ( [[#Haywood--2013|Haywood et al., 2013]] , 2016b) because Earth’s orbit during that time was very similar to that of the modern day.

Further back in time, in the Early Eocene (Cross-Chapter Box 2.1), uncertainties in forcing and temperature change become larger, but the signals are generally larger too ( [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Shaffer--2016|Shaffer et al., 2016]] ; [[#Inglis--2020|Inglis et al., 2020]] ). Caution must be applied when estimating ECS from these time periods, due to differing continental position and topography/bathymetry ( [[#Farnsworth--2019|Farnsworth et al., 2019]] ), and due to temperature-dependence of feedbacks ( [[#7.4.3|Section 7.4.3]] ). On even longer time scales of the last 500 million years ( [[#Royer--2016|Royer, 2016]] ) the temperature and CO <sub>2</sub> measurements are generally asynchronous, presenting challenges in using this information for assessments of ECS.

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==== 7.5.3.4 Synthesis of ECS Based on Paleo Radiative Forcing and Temperature ====

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The lines of evidence directly constraining ECS from paleoclimates are summarized in Table 7.11. Although some of the estimates in Table 7.11 are not independent because they use similar proxy records to each other (e.g., [[#von%20der%20Heydt--2014|von der Heydt et al., 2014]] ; [[#Köhler--2015|Köhler et al., 2015]] , 2017; [[#Stap--2019|Stap et al., 2019]] ), there are still multiple independent lines of paleoclimate evidence regarding ECS, from differing past time periods: LGM ( [[#Sherwood--2020|Sherwood et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ); glacial–interglacial ( [[#Royer--2016|Royer, 2016]] ; [[#Köhler--2017|Köhler et al., 2017]] ; [[#Snyder--2019|Snyder, 2019]] ; [[#Friedrich--2020|Friedrich and Timmermann, 2020]] ); Pliocene ( [[#Martínez-Botí--2015|Martínez-Botí et al., 2015]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ); and the Eocene ( [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Shaffer--2016|Shaffer et al., 2016]] ; [[#Inglis--2020|Inglis et al., 2020]] ), with differing proxies for estimating forcing (e.g., CO <sub>2</sub> from ice cores or boron isotopes) and response (e.g., global surface temperature from δ <sup>18</sup> O, Mg/Ca or Antarctic δ D). Furthermore, although different studies have uncertainty estimates that account for differing sources of uncertainty, some studies ( [[#Snyder--2019|Snyder, 2019]] ; [[#Inglis--2020|Inglis et al., 2020]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ) do consider many of the uncertainties discussed in Sections 7.5.3.1–7.5.3.3. All the studies based on glacial–interglacial cycles account for some aspects of the state-dependence of climate sensitivity ( [[#7.4.3|Section 7.4.3]] ) by considering only the warm phases of the Pleistocene, although what constitutes a warm phase is defined differently across the studies.

None of the post-AR5 studies in Table 7.11 have an estimated lower range for ECS below 1.6°C. As such, based solely on the paleoclimate record, it is assessed to be ''very likely'' that ECS is greater than 1.5°C ( ''high confidence'' ).

In general, it is the studies based on the warm periods of the glacial–interglacial cycles ( [[#7.5.3.2|Section 7.5.3.2]] ) that give the largest values of ECS. Given the large uncertainties associated with estimating the magnitude of the ice-sheet forcing during these intervals ( [[#Stap--2019|Stap et al., 2019]] ), and other uncertainties discussed in ( [[#7.5.3.2|Section 7.5.3.2]] , in particular the direct effect of orbital forcing on estimates of ECS, there is only ''low confidence'' in estimates from the studies based on glacial–interglacial periods. This ''low confidence'' also results from the temperature-dependence of the net feedback parameter, α , resulting from several of these studies (Figure 7.10), that is hard to reconcile with the other lines of evidence for α , including proxy estimates from warmer paleoclimates ( [[#7.4.3.2|Section 7.4.3.2]] ). A central estimate of ECS, derived from the LGm ( [[#7.5.3.1|Section 7.5.3.1]] ) and warm periods of the pre-Quaternary ( [[#7.5.3.3|Section 7.5.3.3]] ), that takes into account some of the interdependencies between the different studies, can be obtained by averaging across studies within each of these two time periods, and then averaging across the two time periods; this results in a central estimate of 3.4°C. This approach of focussing on the LGM and warm climates was also taken by [[#Sherwood--2020|Sherwood et al. (2020)]] in their assessment of ECS from paleoclimates. An alternative method is to average across all studies, from all periods, that have considered multiple sources of uncertainty (Table 7.11); this approach leads to a similar central estimate of 3.3°C. Overall, we assess ''medium confidence'' for a central estimate of 3.3°C to 3.4°C.

There is more variation in the upper bounds of ECS than in the lower bounds. Estimates of ECS from pre-Quaternary warm periods have an average upper range of 4.9°C, and from the LGM of 4.4°C; taking into account the independence of the estimates from these two time periods, and accounting for state-dependence ( [[#7.4.3|Section 7.4.3]] ) and other uncertainties discussed in ( [[#7.5.3|Section 7.5.3]] , the paleoclimate record on its own indicates that ECS is ''likely'' less than 4.5°C. Given the higher values from many glacial–interglacial studies, this value has only ''medium'' ''confidence'' . Despite the large variation in individual studies at the extreme upper end, all except two studies (both of which are from glacial–interglacial time periods associated with ''low confidence'' ) have central estimates that are below 6°C; overall we assess that it is ''extremely likely'' that ECS is below 8°C ( ''high confidence'' ).

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'''Table 7.11''' '''|''' '''Estimates of equilibrium climate sensitivity (ECS) derived from paleoclimates; from AR5 (above double lines) and from post-AR5 studies (below double lines).''' Many studies provide an estimate of ECS that includes only CO <sub>2</sub> and the ice-sheet feedback as forcings, providing an estimate of S <sub>[CO2, LI]</sub> using the notation of [[#Rohling--2012|Rohling et al. (2012)]] , which is equivalent to our definition of ECS (Box 7.1). However, some studies provide estimates of other types of sensitivity (column 4). Different studies (column 1) focus on different time periods (column 2) and use a variety of different paleoclimate proxies and models (column 3) to give a best estimate (column 5) and/or a range (column 5). The published ranges given account for varying sources of uncertainty (column 6). See Cross-Chapter Box 2.1 for definition of time periods. All temperature values in column 5 are shown to a precision of 1 decimal place.

{| class="wikitable"
|-
| (1) Study

| (2) Time Period (kyr = thousand years; Myr = million years; Ma = million years ago)

| (3) Proxies/Models Used for CO <sub>2</sub> , Temperature (T) and Global Scaling (GS)

| (4) Climate Sensitivity Classification According to [[#Rohling--2012|Rohling et al. (2012)]]

| (5) Published Best Estimate of ECS [and/or Range]

| (6) Range Accounts For:

|-
| AR5 ( [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] )

| LGM (Last Glacial Maximum)

| Assessment of multiple lines of evidence

| S <sup>a</sup> = ECS a

| [ ''very likely'' &gt;1.0; ''very unlikely'' &gt;6.0°C]

| Multiple sources of uncertainty

|-
| AR5 ( [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] )

| Cenozoic (last 65 Myr)

| Assessment of multiple lines of evidence

| S <sub>[CO2,LI]</sub>

| [95% range: 1.1°C to 7.0°C]

| Multiple sources of uncertainty

|-
| [[#Tierney--2020b|Tierney et al. (2020b)]]

| LGM

| CO <sub>2</sub> : ice core

T: multi-proxy

| S <sub>[CO2,LI,CH4, N2O]</sub>

| 3.8°C

[68% range: 3.3°C to 4.3°C]

| Multiple sources of uncertainty

|-
| [[#Sherwood--2020|Sherwood et al. (2020)]]

| LGM

| CO <sub>2</sub> : ice core

T: multiple lines of evidence

| S <sub>[CO2, LI, CH4, N2O, dust, VG]</sub>

| maximum likelihood [likelihood of 1.0]: 2.6°C

[ ''likely'' range depends on chosen prior; likelihood of 0.6: 1.6°C to 4.4°C]

| Multiple sources of uncertainty

|-
| [[#von%20der%20Heydt--2014|von der Heydt et al. (2014)]]

| Warm states of glacial–interglacial cycles of last 800 kyr

| CO <sub>2</sub> : ice core

T: ice core δ D, benthic δ <sup>18</sup> O

GS: [[#Schneider%20von%20Deimling--2006|Schneider von Deimling et al. (2006)]] ; [[#Annan--2013|Annan and Hargreaves (2013)]]

| S <sub>[CO2,LI]</sub>

| 3.5°C

[range: 3.1°C to 5.4°C] <sup>b</sup>

| Varying LGM global mean temperatures used for scaling

|-
| [[#Köhler--2015|Köhler et al. (2015)]]

| Warm states of glacial–interglacial cycles of last 2 Myr

| CO <sub>2</sub> : ice core alkenones and boron isotopes

T: benthic δ <sup>18</sup> O

GS: PMIP LGM and PlioMIP MPWP

| S <sub>[CO2,LI]</sub>

| 5.7°C

[68% range: 3.7°C to 8.1°C] <sup>b</sup>

| Temporal variability in records

|-
| [[#Köhler--2017|Köhler et al. (2017)]]

| Warm states of glacial–interglacial cycles of last 2 Myr

| CO <sub>2</sub> : boron isotopes

T: benthic δ <sup>18</sup> O

GS: PMIP LGM and PlioMIP MPWP

| S <sub>[CO2,LI]</sub>

| 5.6°C

[16th to 84th percentile: 3.6°C to 8.1°C] <sup>b</sup>

| Temporal variability in records

|-
| [[#Köhler--2018|Köhler et al. (2018)]]

| Warm states of glacial–interglacial cycles of last 800 kyr, excluding those for which CO <sub>2</sub> and T diverge

| CO <sub>2</sub> : ice cores

T: benthic δ <sup>18</sup> O, alkenone, Mg/Ca, MAT, and faunal SST

GS: PMIP3 LGM

| S <sub>[CO2, LI]</sub>

| [range: 3.0°C to 5.9°C] <sup>b</sup>

| Varying temperature reconstructions

|-
| [[#Stap--2019|Stap et al. (2019)]]

| States of glacial–interglacial cycles of last 800 kyr for which forcing is zero compared with modern, excluding those for which CO <sub>2</sub> and T diverge

| CO <sub>2</sub> : ice cores

T: benthic δ <sup>18</sup> O

GS: PMIP LGM and PlioMIP MPWP

| S <sub>[CO2, LI]</sub>

| [range: 6.1°C to 11.0°C] <sup>b</sup>

| Varying efficacies of ice-sheet forcing

|-
| (1) Study

| (2) Time Period (kyr = thousand years; Myr = million years; Ma = million years ago)

| (3) Proxies/Models Used for CO <sub>2</sub> , Temperature (T) and Global Scaling (GS)

| (4) Climate Sensitivity Classification According to [[#Rohling--2012|Rohling et al. (2012)]]

| (5) Published Best Estimate of ECS [and/or Range]

| (6) Range Accounts For:

|-
| [[#Friedrich--2016|Friedrich et al. (2016)]]

| Warm states of glacial–interglacial cycles of last 780 kyr

| CO <sub>2</sub> : ice cores

T: alkenone, Mg/Ca, MAT, and faunal SST

GS: PMIP3 LGM

| S <sub>[GHG,LI,AE]</sub>

| 4.9°C

[ ''Likely'' range: 4.3°C to 5.4°C] <sup>b</sup>

| Varying LGM global mean temperatures, aerosol forcing

|-
| [[#Friedrich--2020|Friedrich and Timmermann (2020)]]

| Last glacial–interglacial cycle

| CO <sub>2</sub> : ice cores

T: alkenone, Mg/Ca, MAT

| S <sub>[GHG,LI,AE]</sub>

| 4.2°C

[range: 3.4°C to 6.2°C] <sup>b</sup>

| Varying aerosol forcings

|-
| [[#Snyder--2019|Snyder (2019)]]

| Interglacial periods and intermediateglacial climates of last 800 kyr

| CO <sub>2</sub> : ice cores

T: alkenone, Mg/Ca, species assemblages

GS: PMIP models

| S <sub>[GHG,LI,AE,VG]</sub>

| 3.1°C

[67% range: 2.6°C to 3.7°C] <sup>b</sup>

| Multiple sources of uncertainty

|-
| [[#Royer--2016|Royer (2016)]]

| Glacial–interglacial cycles of the Pliocene (3.4 to 2.9 Ma)

| CO <sub>2</sub> : boron isotopes

T: benthic δ <sup>18</sup> O

| S <sub>[CO2,LI]</sub>

| 10.2°C

[68% range: 8.1°C to 12.3°C]

| Temporal variability in records

|-
| [[#Martínez-Botí--2015|Martínez-Botí et al. (2015)]]

| Pliocene

| CO <sub>2</sub> : boron isotopes

T: benthic δ <sup>18</sup> O

| S <sub>[CO2,LI]</sub>

| 3.7°C

[68% range: 3.0°C to 4.4°C] <sup>b</sup>

| Pliocene sea level, temporal variability in records

|-
| [[#Sherwood--2020|Sherwood et al. (2020)]]

| Pliocene

| CO <sub>2</sub> : boron isotopes

T: multiple lines of evidence

| S <sub>[CO2, LI,N2O,CH4,VG]</sub>

| maximum likelihood [likelihood of 1.0]: 3.2°C

[ ''likely'' range depends on chosen prior; likelihood of 0.6: 1.8°C to 5.2°C]

| Multiple sources of uncertainty

|-
| [[#Anagnostou--2016|Anagnostou et al. (2016)]]

| Early Eocene

| CO <sub>2</sub> : boron isotopes

T: various terrestrial MAT, Mg/Ca, TEX, δ <sup>18</sup> O SST

| S <sub>[CO2,LI]</sub>

| 3.6°C

[66% range: 2.1°C to 4.6°C]

| Varying calibrations for temperature and CO <sub>2</sub>

|-
| [[#Anagnostou--2020|Anagnostou et al. (2020)]]

| Late Eocene (41.2 to 33.9 Ma)

| CO <sub>2</sub> : boron isotopes

T: one SST record

GS: CESM1

| S <sub>[CO2,LI]</sub>

| 3.0°C

[68% range: 1.9°C to 4.1°C]

| Temporal variability in records

|-
| [[#Shaffer--2016|Shaffer et al. (2016)]]

| Pre-PETM (Paleocene–Eocene Thermal Maximum)

| CO <sub>2</sub> : mineralogical, carbon cycling, and isotope constraints

T: various terrestrial MAT, Mg/Ca, TEX, δ <sup>18</sup> O SST

| S <sub>[GHG,AE,VG,LI]</sub>

| [range: 3.3°C to 5.6°C]

| Varying calibration of temperature and CO <sub>2</sub>

|-
| [[#Inglis--2020|Inglis et al. (2020)]]

| Mean of EECO (Early Eocene Climatic Optimum), PETM, and latest Paleocene

| CO <sub>2</sub> : boron isotopes

T: multiproxy SST and SAT

GS: EoMIP models

| S <sub>[CO2,LI, VG,AE]</sub>

| 3.7°C [ ''likely'' range: 2.2°C to 5.3°C]

| Multiple sources of uncertainty

|}

a S <sup>a</sup> in this table denotes a classification of climate sensitivity following [[#Rohling--2012|Rohling et al. (2012)]] .

<sup>b</sup> Although our assessed value of ERF due to CO <sub>2</sub> doubling is 3.93 W m <sup>–2</sup> [[#7.3.2.1|Section 7.3.2.1]] ), for these studies the best estimate and range of temperature is calculated from the published estimate of sensitivity in units of °C (W m <sup>–2</sup> ) <sup>–1</sup> using an ERF of 3.7 W m <sup>–2</sup> , for consistency with the typical value used in the studies to estimate the paleo CO <sub>2</sub> forcing.

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